Lymphocytes are a type of white blood cell involved in immune system regulation. There are two broad categories of lymphocytes, namely T cells and B cells. T-cells are responsible for cell-mediated immunity whereas B-cells are responsible for humoral immunity (relating to antibodies). T-cells are named such because these lymphocytes mature in the thymus and B-cells mature in bone marrow. Lymphocytes are much more common in the lymphatic system, and include B cells, T cells, killer T-cells, and natural killer cells. B cells make antibodies that bind to pathogens to enable their destruction. CD4+ (helper) T cells co-ordinate the immune response (they are what become defective in an HIV infection). CD8+ (cytotoxic) T cells and Natural Killer (NK) cells are able to kill cells of the body that are infected by a virus or display an antigenic sequence.
Natural killer cells are CD56(+)CD3(−) large granular lymphocytes that constitute a key component of the human innate immune response. In addition to their potent cytolytic activity, NK cells express a host of immunoregulatory cytokines and chemokines that play a crucial role in pathogen clearance. Furthermore, interactions between NK and other immune cells are implicated in triggering the adaptive, or antigen-specific, immune response.
The interactions between immune and inflammatory cells are mediated in large part by cytokine proteins, for example, lymphokines such as interleukins (IL), which are able to promote cell growth, differentiation, and functional activation. Currently, at least twenty-three interleukins and their various splice variants have been described. Some of these cytokines mediate distinct biological effects but many have overlapping activities. The understanding of interleukin structure and function has led to new and important insights into the fundamental biology of immunity and inflammation. For example, Interleukin-2 (IL-2) and IL-15 are two distinct cytokines with partially overlapping properties that are implicated in the development, homeostasis, and function of T cells and NK cells.
IL-2, formerly referred to as T-cell growth factor, is a powerful immunoregulatory lymphokine that is produced by antigen-activated T cells. It is produced by mature T lymphocytes on stimulation but also constitutively by certain T-cell lymphoma cell lines. IL-2 is useful in the study of the molecular nature of T-cell differentiation, and because it augments natural killer cell activity, it can be useful in modulating the immune response to cancers, viral or bacterial infections. Also, IL-2 can act as a growth hormone for both B and T lymphocytes, and stimulates clonal expansion and maturation of these lymphocytes. IL-2 binds to its receptor (R) complex comprised of IL-2R alpha (“IL-2Ra”), IL-2R beta (“IL-2Rb”), and -gamma (“gC”) chains, and exerts its effect via second messengers, mainly tyrosine kinases, which ultimately stimulate gene expression.
The heterotrimerization of the receptor chains leads to high affinity binding for IL-2. The functional importance of IL-2Ra in hematopoietic cell systems is well known. However, the potential role that IL-2Ra plays in tumorigenesis is still not fully elucidated. IL-2Ra expression has been found in many types of cancers, including leukemia, lymphoma, lung, breast, head-and-neck, and prostate. Also, high expression of IL-2Ra in tumors correlates with a poor prognosis for the patient.
IL-15 is a member of the four alpha-helix bundle family of lymphokines and its mRNA can be detected in a wide variety of tissues of both non-hematopoietic, and hematopoietic lineages but it is not produced by T cells. IL-15 is difficult to detect at the protein level in vivo perhaps due to short protein half-life and tight transcriptional and translational control. IL-15 is a soluble protein made by many cells in the body which play an important role in the development of the immune system. IL-15 was simultaneously discovered in an adult T-cell leukemia cell line and a simian kidney epithelial cell line as a 14 kDa-16 kDa protein able to stimulate cytotoxic T cell lymphocyte cell line (CTLL) and peripheral blood T cell proliferation, and to induce peripheral blood mononuclear cells to exhibit effector function.
IL-15 plays a multifaceted role in development and control of the immune system. More specifically, IL-15 influences the function, development, survival, and proliferation of CD8+ T cells, NK cells, killer T cells, B cells, intestinal intraepithelial lymphocytes (IEL) and antigen-presenting cells (APC). It has been demonstrated that both IL-15−/−, and IL-15Ra−/− transgenic mice lack peripheral NK and killer T cell populations, certain IEL subsets, and most memory phenotype CD8+ T cells. In addition, while antigen-specific memory CD8+ T cells can develop in response to pathogens in both types of knockout mice, the resulting memory CD8+ T cell pool undergoes dramatic erosion over time. Suggesting a crucial role for IL-15 in mediating long term memory CD8+ T cell proliferation and survival.
The IL-15 receptor (R) consists of three polypeptides, the type-specific IL-15R alpha (“IL-15Ra”), the IL-2/IL-15Rbeta (“IL-2Rb”), and the common gamma chain (“gC,” which is shared by multiple cytokine receptors). The high affinity IL-15Ra chain (Kd≈10−11 M) is thought to form a heterotrimeric complex with the shared IL-2Rb, and the gC. Similar to IL-15, IL-15Ra is thought to be expressed by a wide variety of cell types but not necessarily in conjunction with IL-2Rb and gC. Although the IL-15Ra, the IL-2Rb, and the gC chains are believed to associate as a heterotrimeric receptor, whether this is the physiologically relevant form of the IL-15 receptor remains a matter of speculation. For example, the IL-15Ra chain does not co-precipitate with the IL-2Rb/gC in the presence of IL-15.
Moreover, unlike the IL-2Ra chain, the IL-15Ra chain apparently mediates signal transduction. IL-15Ra is a 58-60 kDa protein that shares structural similarities to the IL-2Ra protein. IL-15Ra and IL-2Ra genes also share similar intron-exon organization and are closely linked on human chromosome 10p14-p15. Human IL-15Ra shares about 45% amino acid (aa) homology with the mouse form of the receptor. Eight isofornms of IL-15Ra mRNA have been identified resulting from alternative splicing events involving different exons. The exclusion of exon 2 (ΔExon2) results in an IL-15Ra isoform that does not bind IL-15. Human IL-15Ra-ΔExon3 cDNA encodes a 267 amino acid (aa) protein that contains a 30 aa signal sequence, a 175 aa extracellular region containing one N-linked glycosylation site, a 21 aa transmembrane domain and a 41 aa cytoplasmic tail.
IL-15 signaling can occur through the heterotrimeric complex of IL-15Ra, IL-2Rb and gC; through the heterodimeric complex of IL-2Rb and gC; or through a novel 60-65 kDa IL-15RX subunit found on mast cells. (Anderson, D. M. et al., 1995, J. Biol. Chem. 270:29862-29869; Waldemann, T. A. and Y. Tagaya, 1999, Ann. Rev. Immunol., 17:19-49; Dubois, S. et al., 1999, J. Biol. Chem. 274:26978-26984). Recently, the binding of IL-15 to IL-15Ra has been reported to antagonize the TNF-alpha-mediated apoptosis in fibroblasts by competing with TNFRI for TRAF2 binding (Bulfone-Paus, S. et al., 1999, FASEB 13:1575-1585).
Given the known effects of IL-15 on the immune system, a number of groups have proposed targeting IL-15, to manipulate the immune system for the hosts benefit. While IL-15 administration has been employed to bolster immune responses or augment immune system reconstitution, blockade of IL-15 activity can inhibit autoimmune responses. For example, administration of an IL-15-activity blocking mutant IL-15-F protein or a soluble form of the IL-15Ra has therapeutic potential in a mouse model of arthritis and allograft survival.
Conversely, IL-15 (protein or DNA-expression vector) administered as an adjuvant during vaccination or infection augments CD8+ T cell immunity, and IL-15 treatment can enhance protection of mice from lethal doses of Mycobacterium tuberculosis and Escherichia coli. Furthermore, IL-15 therapy stimulates anti-HIV immunity and increases survival of CD4+ and CD8+ lymphocytes from HIV-infected patients in vitro. IL-15 can also accelerate immune reconstitution after bone marrow transplant. Several groups have found that IL-15 therapy, in conjunction with chemotherapy, Toll-like receptor agonists, or adoptive transfer of tumor reactive CD8+ T cells, can result in increased survival or complete tumor regression in mouse tumor models, in contrast to each therapy alone. Thus, manipulation of IL-15 activity has potential as a therapeutic modality in a number of clinical situations.
IL-15 is currently being used in many studies in which augmentation of the immune response is desirable. These include increasing the efficacy of vaccines against tumors and infections as well as augmenting the ability of the body to remove cancers in the absence of overt vaccination. In addition, IL-15 may aid in regenerating the immune system following bone marrow transplant or in AIDS. However, the half-life of IL-15 in vivo is very short (minutes to 1 hour or so) and this is one reason for poor efficacy. At present the only way to obtain any effect of IL-15 activity is by using large doses, and IL-15 alone is not always effective. Researches have attempted to increase the half-life of IL-15 using molecular modifications but these have generally been ineffective. For example, PEGylation (a common technique to increase protein half-life) of IL-15 increases the half-life but destroys the majority of the activity of the cytokine, in fact, PEG-IL-15 is an antagonist of IL-15 activity.
Therefore, there exists an unmet need to provide a suitable therapeutic form of IL-15 that demonstrates a longer half-life, and a greater efficacy at lower dosages when administered to an organism in need thereof for purposes of modulating or enhancing immunity. Such a therapeutic would allow for the administration of less cytokine while simultaneously providing for the augmentation of the hosts immune system beyond the effects of IL-15 alone.
Our studies showed that the IL-15Ra acts to “transpresent” IL-15 to opposing cells expressing the IL-2/15Rb/gC complex without a requirement for IL-15Ra expression. In addition, in vitro, IL-15 bound to a chimera comprised of the soluble portion of the IL-15Ra valenty linked to an antibody Fc region (IL-15Ra-Fc) (R&D Systems, Inc, Minneapolis, Minn.), supports the survival of IL-15Ra−/− memory CD8 T cells, in contrast to either component alone.
It is generally perceived by those in the pertinent art that the soluble portion of the IL-15Ra is an inhibitor of IL-15 action. In fact, published research has demonstrated that IL-15Ra can inhibit IL-15 activity in vitro and in vivo. Presently, no one has yet devised a system in which IL-15 and IL-15Ra are pre-coupled prior to administration as an in vivo treatment.